Where air conditioning or refrigeration systems have varying load requirements, it is common to have a bank of compressors in the refrigeration circuit. The compressors may have different capacities and can be run individually or in various combinations to provide more efficient operation for the varying load requirements. In positive displacement compressor systems, gas pulsations, vibrations and sound radiation occur at exact integer multiples of compressor running speed. When two, or more, similar positive displacement compressors are operated in the same refrigerant circuit, or in separate refrigerant circuits in the same equipment room, only a slight difference in running speed causes beats to occur due to the slight differences in the frequencies of these phenomena. The beats are both annoying and potentially damaging. Because compressors most commonly use induction motors as drives, these slight differences will occur.
For induction motors, slip is the difference between the actual motor speed and the synchronous speed and is usually expressed as a percentage of the synchronous speed. Slip is necessary to induce current in the rotor conductors. In going from a no load to a full load condition an induction motor will have about a three to ten percent drop in speed.